Gaseous discharge tube



1940- K. J. GERMESHAUSEN 2,1 5,189

GASEDUS DISCHARGE TUBE Filed Nov. '7, 1935 2 Sheets-Sheet 1 @e7ta 7 a,Emwb cz ermzeshazl-ierz m Jan. 2, 1940. K. J. GERMESHAUSEN GASEOUSDISCHARGE TUBE Filed Nov. '7, 1935 2 Sheets-Sheet 2 Patented Jan. 2,1940 UNITED STATES PATENT OFFICE Application November 7, 1935, SerialNo. 48,669

13 Claims.

The present invention relates to cold-cathode, gaseous-discharge tubesof the type in which the source of electrons is a bright spot on thesurface of the cathode material, called a cathode spot.

In one well-known tube of this type, the cathode is a'pool of mercury.This tube can pass very high currents, and serves as a good lightsource. It has some disadvantages, which are common to most cold-cathodetubes, among which are "that the cathode spot is diflicult to form andrequires.

complicated equipment to start it, and that the operatingcharacteristics and the light emission of the tube vary with theoperating temperature. Further, the tube cannot be inverted withoutdisplacing the mercury.

An object of my invention is to provide an improved, cold-cathode tubeamong the advantages of which are relatively easy, cathode-spotformation; stability of operating characteristics under changes oftemperature, stray fields, changes of load, etc.; simplicity of control;a solid cathode construction which permits operation of the tube in anyposition; and excellent light emission. Other objects 'will be explainedhereinafter and will be particularly pointed out in the appended claims.

For a consideration of what I believe'to be novel and my invention,attention is directed to the accompanying description and the claimsappended thereto.

In the accompanying drawings, Fig. 1 is a longitudinal section of agaseous discharge tube embodying my invention; Fig. 2 is an elevation ofanother tube embodying my invention; Fig. 3 is a circuit showing onemanner of using the tube; Fig. 4 is a sectional elevation of amodification of the tube; Figs. 5 and 6 show modifications of thecathode and grid structures; Fig. '7 shows a modification of the tube inwhich the grid is adjacent the anode; and Fig. 8 shows a tube in whichthere is a continuous glow discharge to the cathode and in which thestart of the current flow from the anode is controlled by a grid.

' Referring to the drawings, the tube shown in Fig. 1 comprises anevacuated glass envelope I filled with a suitable gas such as neon, andcontaining a cathode 2, an inner grid 3, an outer grid 4 and an anode 5.

The cathode comprises a metal disc 6 secured within and closing thelower end of a cylinder 1 of insulating material. On the upper side ofthe disc is a depression in which is a pill 8 of compressed caesiumchloride and aluminum'filings or powder which is held in place by a wiremesh "the cathode.

screen 9. The pill forms the active material of During the manufactureof the tube, the cathode is heated so that it evolves a small quantityof caesium. The upper end of the cylinder '1 has an inwardly projectingflange 5 It] on which rests a wire-mesh screen which forms the grid 3.The grid 3 is placed with reference to the cathode so that a coating ofcaesium is continually formed thereon during the operation of the tube.The grid 4 is a cylinder 1 of carbon concentric with the cathode. Thelead to the anode is enclosed by insulation a.

In the operation of the tube, a voltage is applied between the grid 4and the grid 3 which is sufficient to cause'a glow discharge betweenthese electrodes. The voltage at which this discharge takes place islow, due to the coating of caesiumv on the grid 3 and the close spacingof the grids. This voltage is also substantially constant, due to thecaesium-coated surface of the grid 3 and the shape of the grids, whichmakes the operation independent of charges on the glass and strayfields. The grid 3 shields the cathode from the grid 4 so that changesin the surface of the cathode do not affect the starting of the glowdischarge between the grids. To start the glow discharge between thegrids requires a voltage of about a hundred volts and a current of afraction of a milliampere, sovery little power is required.

Immediately after the glow discharge starts between the grids, currentflows to the cathode in the form of a glow discharge. When this currentflow reaches sufiicient density at the surface of the cathode, a cathodespot is formed on the surface of the pill 8, causing anarc-characteristic discharge to the cathode. The action of the cathodespot causes the caesium chloride to break down, aluminum beingsubstituted for the cae sium and forming aluminum chloride. .Thisreaction results in the formation of a thin surface layer or coating ofcaesium on the pill which has a great tendency to adhere to the surface.It is on this layer that the cathode spot is formed.

The current passes between the anode and the cathode continually butinterruptedly to set up continual discharges of arc characteristicbetween the cathode and the anode. It is of amperage sufficiently highto produce a potential gradient on a relatively small area only of thecathode high enough to extract electrons from the cathode while thecathode remains "cold, at low average temperature, and to produce a fallof cathode potential lower than that occurring in a glow discharge. Theaverage value of said current is nevertheless relatively low. It isdecreases the sputtering.

sufiiciently low, indeed, so that its root-meansquare value is lowenough so that the average ficiently large so that the averagetemperature of the cathode remains low enough to prevent rapiddisintegration of the cathode. Under the action of the continually butinterruptedly produced cathode spot, the surface of the pill 8 undergoeschanges and there is some sputtering of caesium particles or boiling ofcaesium vapor, but most of this condenses on the grid 3 and on thescreen 9. The adherence of the layer of caesium There is very littlecondensation of caesium on the surface of the carbon grid 4 because thecaesium does not stick readily to carbon or because the caesium isabsorbed into the carbon.

The material of the pill 8, which provides the active material of thecathode, furnishes enough free caesium so that advantage can be taken ofthe ease of spot formation on caesium, which is a material of lowwork-function. If the pill 8 furnished too much free caesium, or if itwere made of pure caesium, or if the arc discharge between the cathodeand the anode were nonintermittent, the cathode spot could be formed,but the caesium would be quickly sputtered and evaporated over the tubeand the cathode would have very short life. As a matter of fact, thecathode spot is more easily formed on the pill of caesium chloride andaluminum than on pure caesium. This is due to the fact that the caesiumchloride and aluminum are in effect surface impurities orirregularities. Viewed from one aspect, the pill 8 is composed of achemical compound of caesium which is slowly broken down under theaction of the cathode spot, liberating free caesium. Viewed from anotheraspect, the pill is composed of a mixture of caesium with a materialwhich retards the vaporization of the caesium under the action of thecathode spot.

Other materials than caesium chloride and aluminum may be used for thepill 8; for example, mixtures of caesium chloride and cadmium or zinc,and mixtures of caesium chloride and rubidium chloride and aluminum,cadmium or zinc. Other metals than caesium, such as the alkali metals,the alkali earth metals, or the rare earth metals, may be used toprovide the active material of the cathode. Examples of these aremixtures of sodium chloride and lead; mixtures of barium chloride andaluminum or zinc; barium oxide; strontium oxide; and misch metal. Alltestshave shown the caesium compounds to be most satisfactory.

In general, a chemical compound of one of the alkali, alkali-earth, orrare-earth, metals mixed with-a metal which will displace the combinedmetal, or a mixture of one of the alkali, alkaliearth, or rare-earth,metals with a substance which will retard the vaporization of the metal,

will be satisfactory and will provide a cathode on and the dischargebetween the anode and cathode will hold over in a glow discharge. Due tothe high cathode drop in glow discharges, the current flow is limited.The form of the screen 9 is not critical as far as the operation of thetube is concerned.

The screen 9 may itself serve as a cathode when a certain amount ofcaesium has been sputtered on its surface. After the cathode spot hasformed on the coating on the screen, the caesium is consumed, and thecathode spot subsequently forms on the pill 8. The action of the cathodeis therefore self regulating, and the operation is not endangered by thesputtering of the caesium.

The form of the arc stream between .the anode and the cathode is acolumn of small diameter which has high intrinsic brilliance, withsubstantially all the light emitted from the concentrated arc stream.This discharge is easily distinguished from a glow discharge whichoccupies substantially the whole interior of the tube, with a dischargeof low intrinsic brilliance, the greatest brilliance being at thesurface of the cathode.

The chimney or restricted passage for the arc discharge provided by thegrid 4 and the cylinder I makes the voltage drop of the arc stream moreconstant and, therefore, increases the stability of the operation of thetube.

There is some tendency for the discharge between the anode and thecathode to become a glow discharge between the outside of the cathodeand the anode. This is prevented by the cylinder 1.. This tendency isfurther resisted-by placing the active material of the cathode at thebottom of a cup or cylinder, since there is a greater tendency for acathode spot to form on an inner surface, such as the bottom of adepression, or in a corner, than there is for the cathode spot to formon an outer surface.

The grid 3 is purposely placed so that caesium will sputter on itssurface during the operation of the tube, and maintain the coating onthe grid. The reason for this is that a glow discharge to a cathodesurface of low work-function starts at a lower voltage. The grid 4 isplaced so that caesium will not be sputtered on its surface. If caesiumdid sputter on its surface, the breakdown voltage of the tube would belowered, due to a tendency of a glow discharge to start between theanode and the grid 4.

Still further to insure that caesium shall not be sputtered on the grid4, it is made of carbon, a material to which the caesium will notreadily adhere. By making the grid 4 of carbon, the

voltage at whicha glow discharge will start bc- The pressure of the gasin the tube may vary. When the tube is used for illumination, pressuresof from 3 to millimeters of neon are suitable.

The tube shown in Fig. 2 is identical with the tube shown in Fig. 1,except that it is provided with a long, restricted passage for the aredischarge. This tube is adapted for higher power applications, andparticularly for uses which require a large amount of light emission.

The advantages of tubes embodying my invention are very clearly shown byits application in the circuit, shown in Fig. 3, as an intermittentlight source for stroboscopic use.

In this circuit the anode 5 and the cathode 2 of the tube are connectedacross a condenser II, which is continuously charged, from a suitablesource, through a variable resistance II. The grid 3 is connected,through resistance I3, to a tap on a resistance l4, connected across thecondenser. The grid 4 is connected, through resistance ii, to a tap on aresistance 16, connected across the condenser. The resistances I 3,

- I4, I! and I8 are large and, therefore, require very little current.The taps on resistances l4 and i8 are proportioned so that the grid 3 ispositive with respect to the cathode 2, and the grid 4 is positive withrespect to the grid 3. In the use of the circuit, the condenser isconnected to a suitable source of direct current and immediately startsto become charged. While the condenser is being charged, the tube isnonconductive. When the condenser reaches-the required voltage, thevoltage between the grid 3 and the grid 4 becomes high enough to cause aglow discharge between the grids. This removes the shielding efiect ofthe grids, and causes a glow discharge to start between the anode andthe cathode, which forms a cathode spot on the cathode and causes an arcdischarge. The discharge of the condenser causes a current of a fewhundred or more amperes to flow through the tube, producing a brightflash of light, which lasts a few microseconds. The resistance l2 doesnot permit the flow of sufficient current to maintain the arc discharge;after the discharge of the condenser, therefore, the arc is extinguishedand the tube becomes non-conducting. The voltage at which the glowdischarge between the grids takes place, and the voltage drop in the arcdischarge, are so nearly constant that the light flashes from the tubeoccur at substantially constant frequency, and can be used forstroboscopic purposes. The rate at which the tube flashes can becontrolled by changing the variable resistance I2 or by changing thebiasing voltage of either grid.

In order to illustrate the possibilities of the tube, the followingfigures are given for a tube having the external dimensions of a smallradio tube:

Condenser discharge voltage 300 volts Peak discharge current 500 amps.

Voltage between grids required to start glow discharge between grids 100volts Current flow in glow discharge be- Less than 1 tween grids.milliamp.

It. In this case, the maximum voltage of the condenser is limited to avalue below the breakdown voltage of the tube, or the bias of the gridsis adjusted so that the tube will not become conducting.

The grid-controlled, cold-cathode, gas-filled, arc-discharge tube thatis herein described has many other uses than as a stroboscope. Itsunique properties can be applied to a number of applications where theability of the tube to pass large currents, to act as a rectifier, tostand ready to operate instantly without requiring cathodepower orcathode-heating time, and to act as a sensitive grid-controlled relay,are important. Those skilled in the art will appreciate that the tubedescribed herein can be used in practically all of the circuits in whichgrid-controlled, hotcathode, gas-filled tubes, such as thyratronsgrid-glow tubes, are now used.

Figs. 4 to 8 inclusive show modifications of the structure of the tube.In the tube shown in Fig. 4, the cathode 2 comprises a metal cup 20, atthe bottom of which is the pill 8 and the screen 9. The outside of thecup 20 is coated with insulating material, which prevents glow dischargeto the outside of the cup in the same manner as the cylinder 1 ofinsulating material shown in Fig. 1. ted in this tube, and the grid 4 ismerely a wire above the end of the cup 20. In the operation of thistube, the glow discharge starts between the grid 4 and the cathode. Thiscauses a glow discharge between the anode 5 and the cup 20, whichreaches sufficient local current density on the surface of the pill 8 toform a cathode spot and start the arc discharge between the anode andthe cathode. In other respects, the operation of the tube issubstantially the same as the tube shown in Fig. l. The glow dischargewhich starts the operation of this tube may be formed between the grid 4and the upper edge of the cup 20, between the grid and, the caesiumcoating which is sputtered on the inside of the cup 20 during theoperation of the tube, or between the grid and the surface of the pill8. Since the cathode is subject to violent, surface changes under theaction of the cathode spot, the voltage at which this glow dischargefrom the grid starts is not so constant as in the tube shown'in Fig. 1.If the cup 20 is deep, the side walls of the cup act as a shield," andthe glow discharge from the grid is not likely to form between the gridand the surface of the pill 8. When the grid .4 is in the form of awire, furthermore, the field between the grid and the cathode, orthemetal cup 20, is not so effectively shielded from charges which inayaccumulate on the glass envelope, and these charges may affect thevoltage required to start the glow discharge from the grid. Theoperation of this tube is, therefore, subject to two variable factors;the surface of the pill 8, and the effect of charges which accumulate onthe glass.

In certain applications, the grid 4 may be completely eliminated. Inthis case, the initial discharge in the tube is a glow discharge betweenthe anode and the cathode, which is followed by an arc discharge.

is held by the screen 9 on the bottom of a metal cup 2|. Across theupper end of the cup is a metal disc 22, having a cylinder 23 secured inan opening 24 in its center. A disc 25 of insulat- One of the grids isomit- This tube is still further affected by charges on the glass, sothat the voltage The grid 4 is in the form of an annular ring having adepending flange on its outer edge, and closely spaced from the upperend of the cylinder 23. The operation of this tube is similar to that ofthe tubes shown in Figs. 1, 2 and 4. The in-v itiating glow dischargestarts between grid 4 and the cylinder 23. The grid 4 shields thecylinder 23 from the charges on the glass, so that the voltage at whichthe glow discharge from the grid starts is relatively constant. Thecylinder 23 provides a restricted opening to the cup 2|, and therebytends to confine the sputtering of the free caesium of the cathode tothe inner surface of the cup 2| and the disc 22. The cylinder 23 alsoprovides a restricted passage for the arc stream, which makes the tubedrop more constant.

In the construction shown in Fig. 6, the oathode and the grids aresupported by a cylinder 32 of insulating material. The cathode comprisesa metal cup 33, secured in the lower end of the tube, and having a pill8 held in the bottom thereof by the screen 9. The cylinder 32 isprovided with an inwardly projecting flange 34, on the inner side ofwhich is secured a metal cylinder 35, which serves as a grid. Thiscylinder is placed so that caesium will sputter on its surface duringthe operation of the tube. The.

other grid comprises a metal cylinder 36, which is secured in the upperend of the cylinder 32. The cylinder 36 is placed so that caesium doesnot sputter on its surface. In the operation of this construction, theinitiating glow discharge takes place between the cylinder 36 and thecoating on the cylinder 35. The voltage at which this glow dischargetakes place is substantially constant, due to the fact that the grids 35and 36 shield the field between the grids from charges on the glass. Thegrids 35 and 36 also provide a restricted passage for the arc dischargewhich results in substantially constant tube drop.

Thetube shown in Fig. 7 is a modification of the tube shown in Fig. 4,in which the grid 40 is near the anode. The grid 40 has, at its center,an opening 4|, through which the arc discharge passes. The grid 40shields the field between it and the anode from the effect of charges-onthe glass. In the operation of this tube, the initiating glow dischargetakes place between the grid 40 and the anode, and subsequent operationof .the tube is the same as in the previously described tubes. I

In the tube shown in Fig. 8, the cathode comprises a metal cup 42,having the pill 8 held in the bottom thereof by the screen 9. A metaldisc 43, having an opening 44 at its center, is secured across the upperedges of the cup. Above the disc 43, is a disc 45 of insulatingmaterial, having a metal cylinder 46, projecting through an opening 41in its center. Projecting through the side of the metal cylinder 46, isan electrode 48, which is surrounded by insulation 49. The electrode 48is connected to the anode through a resistance- 50, and the potentialbetween the electrode 48 and the cathode is such that a continuous glowdischarge is formed between these electrodes.

The cylinder 46 serves as-a grid and is normallybi'ased, so thatdischarge between the anode and cathode is prevented. When the bias onthe control grid 46 is changed to the proper value, a glow dischargetakes place between the anode and the cathode, which results in theformation of a cathode spot, and thereby produces an arc dischargebetween the anode and the cathode. The operation of this tube islikewise unafiected by charges on the glass due to the shielding effectof the metal cylinder 46*. In a tube of this construction, it issatisfactory to have a continuous glow discharge between any twoelectrodes, with some form of shielding to prevent discharge between theanode and the cathode.

Having now described my invention, I claim:

1. A gaseous-discharge tube comprising. an anode, a cathode of amaterial which will break down under the action of a cathode spot andform a surface coating thereon ofa material of low work-function, and ascreen over the cathode material for providing points at which a glowdischarge to the cathode may concentrate on the surface of the cathode,whereby the formation of a cathode spot on the cathode is facilitated.

2. A gaseous-discharge tube comprising an ranged so that the fieldtherebetween is shielded from stray fields, and a glow dischargeinitiated between the anode and cathode by a glow discharge between thegrid and the surface being effective to form a cathode spot on thecathode.

3. A gaseous-discharge tube comprising an anode, a cathode of a materialwhich will break down under the action of a cathode spot and form asurface coating thereon of a material of low work-function, a surfacearranged. so that the material of low work-function on the surface ofthe cathode will be sputtered thereon during the operation of the tube,and a surface adjacent said first surface, a glow discharge between saidsurfaces being adapted to cause an are discharge between the anode andthe cathode.

4. A gaseous-discharge tube comprising an anode, a cathode of a materialwhich will break down under the action of a cathode spot and form asurface coating thereon of a metal of low work-function, an outer grid,and an inner grid arranged to shield the field between the outer gridand the cathode, a glow discharge between said grids being adapted toinitiate an arc discharge between the anode and the cathode.

5. A gaseous-discharge tube comprising an anode, a cathode of a materialwhich will break down under the action of a cathode spot and form asurface coating thereonof a metal of low work-function, an outer grid ofcarbon, and an inner grid arranged so that the metal of lowwork-function will be sputtered thereon during the operation of thetube, and means causing a glow discharge between said grids beingadapted to initiate an arc discharge in the tube.

6. A gaseous-discharge tube comprising an envelope provided with ananode, a cathode of a material which will break down under the action ofthe cathode spot and form a surface coating thereon of a metal of lowwork-function, means in the envelope providing a restricted passageextending from the cathode toward, but not as far as the anode, thecathode occupying a relatively small area in the restricted passage atsome distance away from the end of the passage nearest the anode, and aglow discharge between the anode and the cathode being adapted to form acathode spot on the cathode.

7. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacentto the cathode between the anode and the cathode, thecathode comprising a material that will 76 beark down under the actionof a cathode spot to form on the cathode and on the grid surfacecoatings of the material of a low work-function.

8. A gaseous-discharge device comprising an anode, a cathode and a grid,the cathode comprising a first substance and also a compound containinga second substance of a low work-function, the compound being a materialthat will break down under the action of a cathode spot to form on thecathode a surface coating of the second substance, and the firstsubstance being a material that will replace the second substance in thecompound, whereby the compound will continually break down under theaction of the cathode spot to form the said surface coating on thecathode and the first substance will continually replace the secondsubstance in the compound during the continual breaking down of thecompound under the action of the cathode spot.

9. A. gaseous-discharge device comprising an envelope, an anode, acaesium-containing cathode, an inner grid disposed between the anode andthe cathode, and an outer grid of carbon disposed between the anode andthe inner grid.

10. A gaseous-discharge device comprising an anode, a cathode and a griddisposed adjacent to the cathode between the anode and the cathode, aninsulating cylinder at the lower end of which the cathoderis disposedand at the upper end of which the grid is disposed, the cathodecomprising a material that will break down under the action of a cathodespot to form on the cathode and the grid surface coatings of a materialof a low work-function.

11. A gaseous-discharge device comprising an anode, a cathode, a griddisposed between the anode and the cathode, the cathode comprising a.material that will break down under the action of a cathode spot to formon the cathode a surface coating of a material of low work-function, andcylindrical means for restricting the arc discharge.

12. In a gaseous-discharge device, an insulating cylinder, a metal discclosing the lower end thereof and provided with a depression in itsupper side, a pill of compressed caesium chloride and aluminum filingsor powder in the depression, and a wire-mesh screen covering the pill,the pill constituting a cathode.

13. In a gaseous-discharge device, an insulating cylinder, an anode, ametal disc closing the lower end of the cylinder and provided with adepression in its upper side, a pill of compressed caesium chloride andaluminum filings or powder in the depression, and a wire-mesh screencover ing the pill, the pill constituting a cathode.

KENNETH J. GERMESHAUSEN.

